Combustion chamber for a supercharged direct-injection combustion engine

ABSTRACT

A piston for an internal combustion engine, for example a diesel engine, including a body delimited laterally by a skirt and configured to collaborate with walls of a cylinder of axis of revolution C in which the piston can slide along the axis C, the piston including a transverse face that includes a central pip, a peripheral ring, and a bowl of axis of revolution B that extends from the central pip towards the peripheral ring to which it connects at a lip of thickness Ep. The bowl includes, in substantial vertical alignment with the lip, a torus in profile, for example of a dome-shaped, of maximum radius Rt capable of guiding fuel injected under the lip in a region of a re-entrant zone R towards the central pip. The tip of the pip lying on the axis of revolution B of the bowl rises to a height that is a distance Dt of between 5.4 mm and 8 mm, or substantially equal to 7.2 mm, below the level of the peripheral ring.

BACKGROUND

The invention relates in general to the design of combustion engines,particularly compression-ignition internal combustion engines.

Emissions standards imposed on motor vehicle manufacturers are becomingincreasingly severe, each change in a standard resulting in significanttechnical developments and in the use of additional and/or morecomplicated emissions-control devices which prove to be expensive.

Future standards are encouraging manufacturers especially to reduce thelevel of exhaust emissions of nitrogen oxides and particulates, notablyin order not to clog the particulate filters too quickly, but themanufacturers are at the same time wishing to increase, or at least tomaintain, the engine performance levels and drivability.

Solutions generally adopted to increase emissions control of engines forexample consist in resorting to advanced post-treatment devices such asparticulate filters or in modifying the exhaust gas recirculationspecifications, the English acronym EGR being widely recognized in thisfield, although this has an adverse impact on engine efficiency.

BRIEF SUMMARY

It is an object of the present invention to propose a combustion chamberthat is improved and that is able notably to reduce the level ofnitrogen oxides and particulates exhaust emissions.

One subject of the invention is a piston for an internal combustionengine, notably for a diesel engine, comprising a body laterallydelimited by a skirt able to collaborate with the walls of a cylinder ofaxis of revolution C in which the piston is able to slide along thisaxis C, said piston comprising a transverse face which comprises acentral pip, a peripheral crown ring and a bowl of axis of revolution Bwhich extends from the central pip toward the peripheral crown ring towhich it is connected at a lip of thickness Ep, said bowl comprising,substantially in vertical alignment with the lip, a torus in profile,preferably half-dome shaped, of maximum radius Rt, able to guide a fuelinjected under the lip in a reentrance zone R toward the central pip,notable in that the top of the pip situated on the axis of revolution Bof the bowl rises up to a height that is a distance Dt of between 5.4 mmand 8 mm, and preferably substantially equal to 7.2 mm below the heightof the peripheral crown ring.

Use of a piston according to the invention notably allows the followingadvantages to be realized:

-   -   the post-treatment systems do not have to be made more        complicated and more expensive;    -   the nitrogen oxides emissions are reduced in order to comply        with emissions control standards.

According to some particular embodiments, the piston has one or more ofthe following features:

-   -   the central pip slopes toward the torus by an angle A, measured        from the axis of revolution B of the bowl in the geometric        direction, of between 65° and 65.7° and preferably substantially        equal to 65°;    -   the bowl is centered in the cylinder, the axis of revolution B        of the bowl coinciding with the axis of revolution C of the        cylinder;    -   the thickness Ep of the lip is between 4 mm and 6 mm and        preferably substantially equal to 5.3 mm;    -   the maximum radius of curvature Rt of the torus is between 5 mm        and 6 mm and preferably substantially equal to 5.2 mm;    -   the lip is situated a distance De/2 from the axis of revolution        B of the bowl, the distance De being between 49.6 mm and 51.5 mm        and preferably substantially equal to 50 mm;    -   the torus is situated a distance Db/2 from the axis of        revolution B of the bowl, the distance Db being between 53.8 mm        and 55.4 mm and preferably substantially equal to 54 mm;    -   the difference in distances, with respect to the axis of        revolution B of the bowl, of the end of the crown ring De/2 and        of the end of the torus Db/2 is substantially equal to 2 mm;    -   the maximum depth P of the bowl is between 14.8 mm and 16.2 mm        and preferably substantially equal to 16.2 mm.

A further subject of the invention is an internal combustion enginedesigned for strict emissions control standards in terms of theemissions of nitrogen oxides and particulates and, more specifically, anengine of the diesel type comprising at least one piston according tothe invention.

This internal combustion engine of the diesel type has a cylinder ofaxis of revolution C the upper end of which is closed by a cylinder headprovided with an underside which contributes to defining a combustionchamber together with the transverse face of the piston the piston bowlof which is centered about an axis of revolution B that coincides withthe axis C, said engine comprising, opening onto the underside of thecylinder head, at least one inlet duct that can be closed off by aninlet valve and at least one exhaust duct that can be closed off by anexhaust valve, a preheater plug and a fuel injector the tip of whichopens into the combustion chamber substantially at the axis ofrevolution of the cylinder C.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become clearlyapparent from the description of the embodiment thereof givenhereinafter by way of entirely nonlimiting indication with reference tothe following figures:

FIG. 1 is a partial schematic cross section of an internal combustionengine according to the invention;

FIG. 2 is a partial cross section on a plane containing the axis ofrevolution of the piston bowl, showing the detail of the crown part of apiston according to the invention, on the inlet side.

DETAILED DESCRIPTION

FIG. 1 illustrates an internal combustion engine 100, notably for adiesel engine, comprising at least one cylinder 1 of axis of revolutionC, a cylinder head 2 and a piston 3.

In the remainder of the description, this axis of revolution C will beconsidered to be oriented upward, toward the cylinder head 2.

The piston 3 is mounted in the cylinder 1 such that it can slide alongthe axis of revolution of the cylinder C, and has a piston body designedto accept a pin to connect the piston to the little end of a drivingconnecting rod, said body being laterally delimited by a piston skirt 31running parallel to the axis of revolution of the cylinder C andcollaborating with an internal wall of the cylinder 1. The pistonfurther comprises a transverse face 32 which, together with theunderside 20 of the cylinder head 2, contributes to delimiting acombustion chamber of the cylinder 1.

The fresh air or a mixture of fresh air and of recirculated exhaustgases is admitted to the combustion chamber via at least one inlet duct5 formed in the cylinder head 2 and which can be closed off by at leastone inlet valve 50.

The residue of combustion of the air-fuel mixture introduced is removedvia at least one exhaust duct 6 formed in the cylinder head 2 and whichcan be closed off via at least one exhaust valve 60.

A preheater plug 4 is fitted in the cylinder head 2, its end openinginto the combustion chamber so that it can heat the air-fuel mixtureduring cold starts.

A fuel injector 7 is fitted in the cylinder head 2 and opens into thecombustion chamber substantially along the axis of revolution C of thecylinder 1.

As depicted in FIG. 2 which, in partial cross section on an axial plane,details the upper part of a piston 3 according to the invention, thetransverse face 32 comprises a central pip 321, a peripheral crown ring322 and an annular cavity or bowl 323 of axis of revolution B thatcoincides with the axis of revolution of the cylinder C, said bowl 323extending from the central pip 321 toward the peripheral crown ring 322to which it is connected.

As an alternative, the bowl 323 may be offset in the cylinder 1, theaxes of revolution C of the cylinder 1 and B of the bowl 323 thereforebeing able to be offset from one another by a distance preferablyshorter than a value substantially equal to 3 mm.

The peripheral crown ring 322 extends laterally from the skirt 31 of thepiston 3 toward the axis of revolution B of the bowl 323 as far as anend that forms a lip 3220 underneath which the bowl 323 has a torus 3230in profile half-dome shaped, with a maximum radius of curvature Rt ofbetween 5 mm and 7 mm and preferably substantially equal to 6 mm.

The lip 3220 is situated a distance De/2 from the axis of revolution Bof the bowl 323, the distance De being between 49.6 mm and 51.5 mm andpreferably substantially equal to 50 mm.

The torus 3230 is situated a distance Db/2 from the axis of revolution Bof the bowl 323, the distance Db being between 53.8 mm and 55.4 mm andpreferably substantially equal to 54 mm.

The injector 7 is designed selectively to inject fuel in the form ofjets directed into an upper region of the torus 3230 adjacent to abottom edge of the lip 3220, also known as a reentrant edge R, so as toimprove the way in which the jet of fuel is guided from this reentrantedge R by rolling along the walls of the torus 3230 toward the bottom ofthe bowl, where the oxygen is to be found as the piston 3 effects itsupstroke, so as to reduce smoke and so as to enable the gases to becirculated toward the central pip 321.

For preference, a reentrant edge R will be maintained by keeping thedifference in distances, with respect to the axis of revolution B of thebowl 323, of the end of the crown ring De/2 and of the end of the torusDb/2 substantially equal to 2 mm.

For preference, the thickness Ep of the lip 3220 corresponding to thedistance of the reentrant edge R of the peripheral crown ring 322 isbetween 4 mm and 6 mm and preferably substantially equal to 5.3 mm.

The maximum depth P of the bowl 323 is between 14.8 mm and 16.2 mm andpreferably substantially equal to 16.2 mm. The ratio Db/P gives the bowl323 a substantial width which allows effective exploitation of the airunder full load conditions, making it possible to achieve high specificperformance, even with low swirl rates thus limiting the risk of thejets emitted by the injector 7 overlapping.

Combining this design of the reentrant edge R, with the torus 3230 andwith a maximum bowl 323 depth P allows improved guidance of the jet offuel toward a volume of air trapped in the bottom of the bowl 323.

The top of the central pip 321, situated on the axis of revolution B ofthe bowl, rises up to a height that is a distance Dt below the height ofthe peripheral crown ring, this distance Dt being between 5.4 mm and 8mm, and preferably substantially equal to 7.2 mm. The distance Dt to thetop of the peripheral crown ring makes it possible to limit interactionsbetween the jets of fuel sprayed toward the bottom of the bowl with thecentral pip 321 and notably the top thereof, thus making it possible toreduce emissions of smoke and unburnt fuel.

The central pip 321 slopes by an angle A, measured from the axis ofrevolution B of the bowl in the geometric direction, of between 65° and65.7° and preferably substantially equal to 65°.

Having the central pip 321 slope at an angle A like this allows the sootformed by combustion at the bottom of the bowl 323 to be extractedtoward the central pip 321 for improved post-oxidation leading to areduction in smoke.

An internal combustion engine 100 provided with a piston 3 combiningthese features proves to be particularly advantageous in use bycomparison with an internal combustion engine provided with a pistonsuited to current emissions control standards, of the Euro 4 type. Thesetests were conducted using an engine 100 that had the followingadditional features: a bore of 85 mm, a compression ratio of about 15and a level of swirl at bottom dead center Nd/N of about 3.

What was actually found was that under part-load conditions and for thesame level of particulate emissions, this engine 100 emittedapproximately 15% less nitrogen oxides.

For the same power and the same temperature before the turbine TAVT, at4000 rpm and at full load, the amount of smoke was lower, and the smokeindex was reduced by a value substantially equal to 0.5 fsn.

The invention claimed is:
 1. A piston for an internal combustion engine,comprising: a body laterally delimited by a skirt configured tocollaborate with walls of a cylinder of axis of revolution (C) in whichthe piston can slide along the axis (C), the piston comprising atransverse face that comprises a central pip, a peripheral crown ringextending laterally from the skirt, and a bowl of axis of revolution (B)that extends from the central pip toward the peripheral crown ring towhich it is connected at a lip of the peripheral crown ring, the bowlcomprising, substantially in vertical alignment with the lip, a torus inprofile, or of half-dome shaped, configured to guide a fuel injectedunder the lip in a re-entrance zone (R) toward the central pip, whereinthe top of the pip situated on the axis of revolution (B) of the bowlrises up to a height that is a distance (Dt) of between 5.4 mm and 8 mmbelow the height of a top of the peripheral crown ring, and wherein thelip is situated a distance (De)/2 from the axis of revolution (B) of thebowl, wherein the torus is situated a distance (Db)/2 from the axis ofrevolution (B) of the bowl, and the distance (Db)/2 of the torus fromthe axis of revolution (B) of the bowl is greater by substantially 2 mmthan the distance (De)/2 of the lip from the axis of revolution (B) ofthe bowl whereby the lip overhangs the torus, and wherein the maximumdepth (P) of the bowl is between 14.8 mm and 16.2 mm.
 2. The piston asclaimed in claim 1, wherein the bowl is centered in the cylinder, theaxis of revolution (B) of the bowl coinciding with the axis ofrevolution (C) of the cylinder.
 3. An internal combustion engine ofdiesel type, comprising at least one piston as claimed in claim
 2. 4.The piston as claimed in claim 1, wherein a thickness (Ep) of the lip isbetween 4 mm and 6 mm.
 5. The piston as claimed in claim 4, wherein thethickness (Ep) of the lip is substantially equal to 5.3 mm.
 6. Thepiston as claimed in claim 1, wherein a maximum radius of curvature (Rt)of the torus is between 5 mm and 7 mm.
 7. The piston as claimed in claim6, wherein the maximum radius of curvature (Rt) of the torus issubstantially equal to 6 mm.
 8. The piston as claimed in claim 1,wherein the distance (De) is between 49.6 mm and 51.5 mm.
 9. The pistonas claimed in claim 8, wherein the distance (De) is substantially equalto 50 mm.
 10. The piston as claimed in claim 1, wherein the distance(Db) is between 53.8 mm and 55.4 mm.
 11. The piston as claimed in claim10, wherein the distance (Db) is substantially equal to 54 mm.
 12. Thepiston as claimed in claim 1, wherein the distance (Dt) is substantiallyequal to 7.2 mm.
 13. The piston as claimed in claim 1 wherein themaximum depth (P) of the bowl is substantially equal to 16.2 mm.
 14. Thepiston as claimed in claim 1, wherein a face of the central pip extendsin a straight line from the top of the pip to the torus such that anangle (A) is formed between the face of the central pip and the axis ofrevolution (B).
 15. The piston as claimed in claim 14, wherein the angle(A) is between 65° and 65.7°.
 16. The piston as claimed in claim 15,wherein the angle (A) is substantially equal to 65°.
 17. The piston asclaimed in claim 14, wherein the angle (A) is to 65°.